The invention relates to electrodes and more specifically to miniaturized reference electrodes for in vivo medical applications.
Those concerned with monitoring various ionic or gaseous species in blood typically need to be able to use a miniaturized electrode in vivo to obtain continuous real-time indications of the factors to be monitored. One method of obtaining the necessary information is to use miniaturized electrochemical sensors consisting of a reference electrode and a species-selective electrode in contact with the blood flow of a patient. The most convenient configuration for the electrode pair is to be mounted in a lumen of a catheter which may then be inserted into the bloodstream of the patient through a vein or an artery.
In the past, each individual electrode of a pair of electrodes typically required an aqueous phase within its lumen. This aqueous phase was necessary to make contact between the electrode and the external environment, as well as to provide a reference solution having a fixed concentration of a particular ion required to establish a stable potential for the electrodes. This created a problem in medical applications because it made it difficult to sterilize the electrical sensors by means of the standard procedures using ethylene-oxide gas. When ethylene-oxide is used in the sterilization procedure for any object containing water, a reaction can occur between the ethylene-oxide gas and the water to form ethylene glycol, which is quite toxic. Obviously, this is highly undesirable for medical applications. Furthermore, the ethylene glycol will change the nature of the electrode reference solution. Thus, a need existed to develop an electrode which is completely anhydrous, and therefore sterilizable, and yet would function satisfactorily.
An example of the prior art is described in U.S. Pat. No. 3,856,649 to Genshaw et al. In this patent, a solid-state electrode is described for determining ion concentrations in an aqueous solution. The electrode includes an electrically conductive inner element with a salt disposed on a surface portion thereof having a cation and an anion. The cation is identical to at least a portion of the inner electrode material. A solid hydrophilic layer is in intimate contact with the salt and includes a water-soluble salt of the anion. It is important to note that in the device described in the Genshaw reference, the soluble salt must be mixed with the hydrophilic layer which may or may not be dried after application, but which must be wet in order for the electrode to operate. This is undesirable because such an electrode cannot be sterilized with ethylene oxide while wet, and if dried for sterilization, requires a "wet-up" time before use. Furthermore, the electrode described by Genshaw et al. includes a polyvinyl-chloride hydrophobic layer which the present inventors have found to be highly unsatisfactory for the fabrication of reference electrodes. Plasticized polyvinyl-chloride is unsatisfactory because it appears to be inherently selective to certain ions, in particular K.sup.+.
Another ion-sensing device for medical applications is described by Band, D. M. and Treasure, T. J. Physiol. (London), 266, 12, 1977 and reprinted in Ion-Selective Electrode Methodology, Vol. II, 1979, 58. This is a potassium-sensing catheter for in vivo use, and consists of a potassium-sensing element and a reference electrode which are both inserted in separate lumens of a single catheter. The reference electrode consists of a silver/silver chloride wire that is immersed in a saturated solution of potassium chloride which fills the lumen and which must be periodically flushed and replaced. Obviously, this is inconvenient for both medical personnel and patients and is an undesirable feature because the accuracy of electrical readings using the electrode will suffer if the potassium-chloride solution is not periodically refreshed. This type of arrangement is typical of most in vivo chemical sensing devices. In view of the limitation of these types of prior art devices, a need existed to provide a chemical sensor for in vivo applications which is completely dry and capable of being sterilized with ethylene-oxide gas.